1. Field of the Invention
The present invention relates to a method for the surface treatment of anodic oxide film of aluminium or aluminium alloy.
2. Description of the Prior Art
It is well known in the art to form an anodic oxide film by subjecting aluminium or aluminium alloy to the electrolytic treatment of anodic oxidation employing acid such as sulfuric acid or oxalic acid as an electrolyte. The anodic oxide film of aluminium or aluminium alloy thus formed by anodic oxidation is usually called "alumite" and is featured by a large number of micropores present therein. It is also known that under a particular treatment condition, for example, at a lower temperature of the electrolytic bath there can be formed so-called hard alumite which has a fine and hard film structure. Because of its high abrasion resistance, such hard alumite has been used widely as abrasion resisting material for those industrial machine parts for which good abrasion resistance and light weight are required.
On the other hand, however, the very feature of fine and hard structure to which the good abrasion resistance of alumite is attributable constitutes an important disadvantage of the hard alumite. The disadvantage is that such hard alumite is not good in fit to a partner member with which the alumite is in frictional contact. The hard alumite is apt to make a cohesion with the partner member, which often causes the trouble of so-called cohesive or binding friction. In particular, at the beginning of abrasion or under a relatively large load there occur "scratch" and "seizure" which accelerate the cohesive friction to a large extent.
To solve the problem it has been proposed that making use of a plurality of micropores present in an anodic oxide film, lubricant is filled in the micropores so that the cohesive friction may be prevented by improving the fitting property of the hard alumite to any frictionable partner member. However, this known method is practically unusable The size of micropores in a film formed by anodic oxidation is extremely small and is generally in the range of from 100-500 A. It is practically impossible to fill such a small size of micropores with solid lubricant such as powder of molybdenum disulfide. It is allowed only to apply the lubricant onto the surface of such anodic oxide film. Anodic oxide films generally used as abrasion resisting material have usually a film thickness ranging from 10.mu. to 200.mu.. Therefore, the micropores also have a depth ranging from 10.mu. to 200.mu. or more. In addition, these micropores sometimes extend meanderingly to form a network structure. All of these facts put a limitation to the kind of lubricant as well as method useful for impregnating the micropores with lubricant for the above mentioned purpose.
Many attempts have been made to impregnate the micropores with various lubricants such as lubricating oil, polyethylene fluoride and colloidal carbon by immersion or electrode position. However, no satisfactory result has been obtained therefrom. This is attributable to the complicate form of micropores mentioned above. The lubricant used can not reach the interior of micropore or, even when the lubricant can enter the pores to some extent, the amount of lubricant with which the micropores are impregnated is too small to produce the desired effect. Another reason is found in lubricant itself. Since, as described above, the kind of lubricant used for impregnation is limited severely, the micropores are obliged to receive such lubricant whose abrasion resisting property is not good.